1. Field of the Invention
The present invention relates generally to fixation devices and, more particularly, to biologically compatible screws and matching drivers.
2. Description of Related Art
Since the beginning of recorded history, mankind has exhibited an insatiable appetite for constructing new devices and repairing broken ones. One meter for measuring the progress of a society through the generations has been the creativeness and craftsmanship of that societies' architects and engineers.
From ancient wooden aqueducts of the Nile, to lightweight compositional structures of the Space Shuttle, to sophisticated artificial joints of modern medicine, a common ingredient has always been the fastener. Whether the fastener is threaded, removable, or integral with the structure, an accepted engineering principle is that the strength of a product is only as great as the product's weakest link.
In the interest of promoting strength above all else, the prior art has primarily endeavored to construct metallic fasteners of varying sizes and shapes for the majority of applications. The strength of the metal fastener, however, is not achieved without costs. For many applications, metal can be relatively heavy, expensive, and subject to corrosion. Metal fasteners generally are not recyclable and, additionally, are neither biocompatable nor resorbable, when used in connection with medical applications.
Threaded, resorbable fasteners have existed in the prior art for medical applications, such as bone repair and regeneration. A typical resorbable fastener comprises a threaded shaft, a head, and an internal socket disposed within the head for accommodating a driver therein. This internal-socket fastener, although biocompatable and resorbable, has suffered from design deficiencies.
Since the prior art resorbable fastener is designed to be secured to bone within the human body, tissue or other debris may be introduced into the internal socket of the fastener. Materials introduced into the internal socket of the fastener can substantially attenuate or eliminate proper operability of the fastener. For example, a surgeon may have difficulty properly fitting a driver into the internal socket of a fastener that has become partially occluded with debris or human tissue. Since resorbable fasteners are generally manufactured having sizes on an order of millimeters, an obstruction of the very tiny internal socket of a fastener may be difficult to remove.
Although resorbable fasteners are inherently not as strong as metal fasteners with regard to rigidity, sheer strength, etc., resorbable fasteners offer very important benefits for medical applications, including biocompatability and resorbability. Since resorbable materials are relatively weak, as compared to the strength of metal, design considerations for resorbable fastener should maximize strength.
The specific internal-socket design of the prior art resorbable fastener does not appear to be particularly suited for medical and other applications where the strength of the non-magnetic fastener should be optimized. When a prior art internal-socket resorbable fastener is firmly secured into bone, for example, the driver may in some instances disrupt (strip) the internal socket of the fastener. Since the internal-socket is positioned along the rotational axis of the resorbable fastener, a very small moment arm must be utilized to rotate the fastener, resulting in the exertion by the driver of a relatively high rotational force onto the walls of the internal socket. The tiny construction of each internal-socket fastener (on the order of millimeters), the relatively weak material, and the relatively small application moment arm of the internal-socket fastener, all contribute to the relative sensitivity of the system. A need has thus existed in the prior art for a non-magnetic fastener having increased strength.
In additional to the limited strength associated with prior art non-magnetic fasteners, prior art non-magnetic fasteners have also suffered from relatively thick heads. As a result of the relatively weak material of the prior art internal-socket fastener, the head of the internal-socket fastener is typically manufactured to have a relatively thick dimension in a direction parallel to the rotational axis of the fastener. The thicker head of the prior art fastener provides a greater surface area for frictional application of torque by the driver upon insertion of the driver into the internal socket of the fastener. The relatively thick head of the prior art fastener, however, can undesirably protrude from the surface within which it is mounted, thus creating an undesirable non-flush surface.
As a result of the relatively small dimensions of the resorbable fasteners in the context of, for example, bone repair and regeneration applications, the tiny resorbable fasteners are prone to being improperly placed into the target structure. The prior art driver does not firmly hold the fastener and, accordingly, may not accurately align the axis of the fastener with the axis of the driver. The fastener can thus be inadvertently cross threaded or otherwise improperly secured within the target structure. Additionally, as a result of the relatively loose fit between the prior art fastener and driver, the fastener may become dislodged from the target structure and/or the driver, before being completely secured within the target structure.
Prior art drivers used to secure fasteners into target structures are typically not disposable. Accordingly, a single driver is used to secure a plurality of fasteners into the target structure or structures. The user is thus required to manually pick up and align each fastener with both the target structure and the driver, before the fastener can be secured within the target structure or structures. Additionally, due in part to the relatively tiny dimensions of the fastener, a user may accidentally obtain a fastener, having a size other than the desired size, and attempt to secure the improperly-sized fastener into the target structure. The process of manipulating the fastener from the operating table into the target structure, accordingly, can be time consuming and subject to human error. In medical applications, the handling of the fastener by the hand of the user and, further, the multiple uses of the driver on a plurality of fasteners, can increase a probability of infection.